[GHC] #16059: checkValidType is defeated by a type synonym

[GHC]

#16059: checkValidType is defeated by a type synonym
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Compiler (Type    |              Version:  8.7
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC accepts       |  Unknown/Multiple
  invalid program                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 But there is nothing fundamentally wrong with `Foo`.  You might write
 {{{
 f :: Foo Int
 }}}
 which means the same as
 {{{
 f :: Eq Int => Int
 }}}
 which is a perfectly legal type.

 `checkValidType` checks for reasons a type might be invalid ''other than''
 its kind.   Are you sure you want type synonyms to hide all the things we
 check thereby?

 TL;DR: I'm suggesting that if it's valid, it should be valid after
 expanding synonyms.  You are suggesting that ''they should offer some
 level of abstraction''. I'm unconvinced.   Exactly what level of
 abstraction?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16059#comment:6>
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